1. Field of Invention
The invention relates to improvements in cranking performance of a hybrid-vehicle (HV) drive system that drives wheels using an internal combustion engine and an electric motor or motors in combination.
2. Description of Related Art
As reducing air pollution and conserving fuel resources have become important issues in recent years, hybrid vehicles, operated by driving wheels using a combination of an internal combustion engine and an electric motor or motors, have attracted much attention. Japanese Laid-open Patent Publication No.11-198669 discloses a hybrid-vehicle drive system. In the system, a first motor-generator is connected in series to a crankshaft of an internal combustion engine so as to constitute a power shaft which is driven by the internal combustion engine and/or the first electric motor-generator as a motor. The power shaft and an output shaft of a second motor-generator are respectively connected to a ring gear and a sun gear of a planetary gear mechanism. A carrier of the planetary gear mechanism is connected to a transmission, thus serving as an output shaft. The above-structured hybrid vehicle drive system allows the motor generator to directly drive the crankshaft of the internal combustion engine by simply applying electric current to the first motor generator. As a result, cranking of the internal combustion engine can be easily performed.
Another hybrid-vehicle drive system has been proposed which is devised of transmission that has generally been required to be disposed between an output shaft of an internal combustion engine and a vehicle axle. In this drive system, an electric motor is adapted to differentially absorb differences in the rotating speed between an output shaft of an internal combustion engine and a vehicle axle. The differences are caused by a variance between the drive torque with respect to the rotating speed required for the vehicle wheel and that obtained from the internal combustion engine. The aforementioned drive system of the hybrid vehicle is schematically shown in FIG. 1.
Referring to FIG. 1, an internal combustion engine 1 is mounted on a vehicle body (not shown). An output shaft or a crankshaft 2 of the internal combustion engine 1 is connected to a carrier 7 of a planetary gear mechanism 3 including a sun gear 4, a ring gear 5, and planetary pinions 6. A first motor-generator (MG1) 8 includes a coil 9 and a rotor 10. The rotor 10 is connected to the sun gear 4 while the coil 9 is supported on the vehicle body. One end of a propeller shaft 11 is connected to the ring gear 5. The planetary gear mechanism 3 serves as a power distribution mechanism so as to distribute power generated by the internal combustion engine via the crankshaft 2 into the planetary gear mechanism 8 and the propeller shaft 11 serving as a main part of a wheel-drive shaft. A second motor-generator (MG2) 12 is connected to an intermediate portion of the propeller shaft 11. The second motor-generator 12 includes a coil 13 supported on the vehicle body and a rotor 14. In the drive system shown in FIG. 1, for example, the rotor 14 is connected to the propeller shaft 11 by engaging a gear 16 rotatably supported on the rotor 14 to a gear 15 of the propeller shaft 11. However, the connection between the rotor 14 and the propeller shaft 11 may be made in an arbitrary manner. The other end of the propeller shaft 11 is connected to a pair of vehicle axles 18 via a differential 17. A wheel 19 is attached to each vehicle axle 18.
In the drive system shown in FIG. 1, as the crankshaft 2 and the carrier 7 rotate synchronously, the rotating speed of the crankshaft 2 and the carrier 7 is designated as “Vc.” As the motor-generator 8 and the sun gear 4 rotate synchronously, the rotating speed of the motor-generator 8 and the sun gear 4 is designated as “Vs”. The ring gear 5, the second electric motor-generator 12, and the wheels 19 rotate relative to one another, each of which corresponds to the vehicle speed. The rotating speeds of each of the ring gear 5, the second motor generator 12 and the wheels 19 differ depending on a ratio of the number of gear teeth of the gear 15 to that of the gear 16, a speed reducing ratio at the differential 17, and diameter of tire. However, in the following description, the rotating speed of the ring gear 5 will be represented as the rotating speed of these elements and designated as “Vr” for convenience.
FIG. 2 shows the relationship among the rotating speed Vc of the internal combustion engine and the rotating speeds Vs, Vr of the two motor generators MG1, MG2 in the drive structure for the hybrid vehicle with the internal combustion engine and two motor-generators combined with the planetary gear mechanism. The rotating speed Vs is derived from the following equation:Vs=(1+1/ρ)Vc−(1/ρ)Vr
where ρ represents the number of gear teeth of the sun gear relative to that of the ring gear (ρ<1), Vc is determined based on the rotating speed of the internal combustion engine and Vr is determined based on the vehicle speed.
Supposing that each torque at the carrier, the sun gear, and the ring gear is designated as Tc, Ts, and Tr, respectively, each torque is equilibrated as follows:Ts:Tc:Tr=ρ/(1+ρ):1:1/(1+ρ)
When any of these elements generates or absorbs the torque, the torque is transferred among those elements until the above relationship is equilibrated.
In the hybrid vehicle including the drive system structured as described above, operations of the internal combustion engine, MG1 and MG2 are controlled by a controller (not shown) based on operational commands given from an operator or a driver and the operating state of the vehicle. More specifically, the controller includes a microcomputer for calculating a target vehicle speed and a target wheel drive torque on the basis of a vehicle operational command issued by the operator and the operating state of the vehicle represented by detection signals of various sensors. The controller also calculates the output current available at a power storage system or the quantity of power required to be generated and supplied to the power storage system based on a charging state of the power storage system. The controller further determines a required operating mode of the internal combustion engine including a suspension state and a required operating mode or power generating mode of MG1 and MG2. The respective operation of the internal combustion engine, MG1 and MG2 is controlled on the basis of the calculated results.
In the above-structured hybrid vehicle drive system, each value of the rotating speed Vc of the internal combustion engine and the rotating speed Vr corresponding to the vehicle speed, and correlation between the respective values can be changed to a greater extent if the change in those values can be absorbed by the rotating speed Vs of MG1. This is why the hybrid vehicle drive system to operate to operate without a transmission. More specifically, the relationship between the rotating speeds Vc and Vr can be freely changed by adjustment of the power distribution mechanism. The hybrid vehicle drive system allows the engine to be operated (Vc>0) even when the vehicle is stopped, and allows the engine to be stopped (Vc=0) even when the vehicle runs ahead (Vr>0). The hybrid vehicle drive system also allows the vehicle to be reversely operated (Vr<0) even when the engine is operated or stopped (Vc≧0).
The rotating speed of MG2 changes depending on the vehicle speed. The charging state of the power storage system is essentially unrelated to the vehicle speed. Therefore, it is difficult to use MG2 as a power generator for charging the power storage system. MG1 mainly serves to charge the power storage system whereas MG2 mainly serves to drive the wheels of the hybrid vehicle.
In the above structured hybrid vehicle drive system shown in FIG. 1, upon start of the internal combustion engine, electricity is applied to MG1 irrespective of whether the vehicle is stopped (Vr=0) or running (Vr>0). Then MG1 is operated in a forward direction until the rotating speed Vs reaches a predetermined value in accordance with the value of the rotating speed Vr such that the engine speed Vc is increased to a speed that allows autonomous start-up of the internal combustion engine.
Assuming that the line connecting Vs, Vc, and Vr in FIG. 2 is compared to a piece of lever, one end of the lever, that is, Vr has to be supported on its lower side so as to raise the point corresponding to the Vc against cranking of the internal combustion engine by raising the other end of the lever, that is, Vs. Vr is supported on its lower side mainly by the torque in the forward driving direction generated by MG2. The cranking support torque may cause insufficiency in the driving torque generated by MG2 for operating the vehicle by driving the axle. Sufficient driving torque can be obtained by increasing the capacity of MG2. However, such capacity increase leads to a cost increase. If MG2 fails to generate sufficient cranking support torque, the cranking torque is transferred to the wheels, thus generating vibration in the vehicle body. As described above, there are various disadvantages of the generally employed drive system in which MG2 mainly serves to generate the cranking torque.